Postmarc

Postmarc is a graphic postprocessor for Cmarc and Pmarc-12 output
files. It runs under Windows and provides a simple, intuitive, and self-documenting
user interface.
Postmarc provides rotatable and zoomable views of the body and
wake, with
color mappings of
pressure, velocity, and Mach number;
vector arrows representing local velocities; pressure distributions along
any
cross-section; and
on- and
off-body streamlines and
wakes. On-body streamline display includes
boundary layer analysis. Data may be displayed for a single output file
or for the
difference between two output files for the same body (for example with
different angles of attack or dimensions).
Rectangular and
cylindrical scan volumes can be displayed in several ways, including spectrum-plotted
planes cutting through the flow field at multiple locations. Color images
may be printed on compatible color printers. Although Postmarc was written
to be intuitive, some
function keys and arrow keys are used for model rotations and framing.

Wakes

Both the initial geometry and animation of time-stepped wakes may be displayed.

This example shows the wake curl up off a wing tip at 5 degrees angle
of attack in a GIF89A animation.

Contouring control

The first button in the contouring group is the contouring/display setup
form. In this form the user sets options pertaining to the display.

To begin with, the user can set the number of colors to be used. The
maximum number is 256, but large palettes may slow display generation
on some systems. Generally, between 20 and 50 colors provide a good compromise
between detail and speed.

The spectrum range may be set by the user or automatically. The useful
range of pressures is from about -1.5 to 1.0, though the band from -0.4
to 0.4 may be of greater practical interest, and is the default. The useful
range of velocities is from 0 to 1.5. Mach number depends on the flight
speed of the model and the value for the speed of sound provided in the
input deck.

Automatic selection is useful only if the input data are free of spurious highs and lows. This is seldom the case. It is
handy, however, for finding the desired range, for instance when an actual velocity, rather than 1.0, has been entered for
VINF in the Cmarc input file.

Spectrum plots may be smoothly contoured ("Fringe contours" on) or each panel may be solidly colored. Fringe
contours process more slowly, but produce a more realistic and intuitively meaningful display.

Panel edges may be displayed or omitted.

Symmetrical half-models can be mirrored about any of the three orthogonal planes. Display time increases for full
models, since the panel count is doubled. The reflection plane of interest is usually the XZ.

Cp, pressure coefficient

The spectrum plot option depicts pressure distribution on the surface
of the body.

Surface flow vectors

The vector ("XYZ") button causes the surface flow velocity for each panel to be displayed as an arrow on a hidden-line
depiction of the body. The direction of the arrow indicates flow direction and its color represents the speed of the flow.
The length of an arrow does not represent the magnitude of velocity; arrow are sized to fit within their panels.

To enhance the visibility of arrows, the color of the model can be set by
selecting View > Solid color from the pull-down menus. RGB values close
to zero make a dark or black model, ones close to 255 make a light model.

Pressure
Distributions

Once a color "spectrum plot" has been drawn, the user may "drag" a line
anywhere along an orthogonal view of the geometry and a 2D graph will
be generated of the Cp (pressure coefficient), Vm (velocity magnitude)
or M (mach number), whichever was contoured, on both near and far surfaces
along the length of the line. Jaggedness in plots reflects the discontinuous
nature of the data, which is due to the paneling.

On-body streamlines and boundary layer analysis

The paths of streamlines can be displayed in relation to the body. On-body
streamlines originate from the centroids of specified panels. Off-body
streamlines originate from points specified in the input file. Lines are
color-coded to indicate local flow velocity.

In this example, the streamline is marked at the location that laminar
flow transitions to turbulent flow. An "x" marks the location. After the
transition occurs, turbulent separation occurs at the end of the streamline.
This information is displayed by text.

Off-body streamlines

Off Body streamlines allow displaying of coefficient or pressure, velocity
magnitude, or Mach number. Streamlines can be arbitrarily defined through
user defined points.

Off Body streamline locations can be cursor-selected for detailed study
of areas of special interest

Differencing

Postmarc will map the differences in Cp, Vm, and Mach between two models.
The models must be identically paneled, but can differ in dimensions and/or
in flight conditions.

The second icon on the tool bar allows you to select a second file to add to or subtract from the first. After you make a selection, all
commands related to pressure, velocity, and Mach number (including pressure distribution graphs, but not including XYZ plots) relate
to the two files together. The appearance of the icon changes, becoming a triangle. To turn off differencing, click on the triangle.

The default operation is second file minus first file. You can select first-minus-second or even first-plus-second by clicking on File >
Second file operator.

Rectangular and cylindrical velocity scans

Flow velocity and direction may be displayed at grids of off-body points
defined in the input file.

POV Ray rendered model

Postmarc includes output of POV Ray files. Some manual editing is required
for a complete file. Simple light sources and view information is included
in the POV Ray file written by Postmarc. The view information is taken
from the current view in Postmarc.

Postmarc writes
VRML files for your browser (such as CosmoPlayer). Both geometry and contour
plotting results can be written in VRML format. A VRML plug-in is required
to view these two VRML examples.

Streamline Function Capability in Postmarc

Postmarc can compute streamlines for any or all panels, and for any boundary
layer parameter, on the basis of a single Cmarc run. The information used
by Postmarc is stored by Cmarc in a file with the extension .PM. A Monte
Carlo type scheme has been implemented in order to efficiently (and automatically)
calculate enough on-body streamlines to cover virtually every panel. This
allows Boundary layer information to be available on every panel (if separation
has not occured) which allows a
viscous drag estimate.

Selecting streamlines in Postmarc is as easy as cursor picking panels
or choosing the automatic algorithm to generate streamlines that cross
all panels. When one or more streamlines has been identified, the
Manage Panel Crossing dialog box is used to select only cursor-picked
panels or ALL panels for processing. Processing of boundary layer data
is optional. If you want a full boundary layer analysis, you must select
Include Boundary Layer Calculation. If you do not supply values for Reynolds
number and kinematic viscosity, Postmarc automatically looks in the .PM
file (from Cmarc) for the values.

Display of
boundary layer characteristics can be done on the streamlines or panel
averaged fringe contour plots.

Friction coefficient may be mapped over the entire model in the form of
a spectrum plot by selecting the
Cf button in the spectrum plot dialog. Other boundary-layer characteristics,
such as
thickness, shape factor,
laminar transition point , and so on, can be displayed on individual streamlines
or as collections of streamlines covering the entire model or panel averaged.